The availability of various types of electronic devices produces a need to have these devices communicate with each other in a manner which is cost efficient and which can be easily implemented. Such communication can occur between two or more user's communication devices (for example, telephones, computers, printers, facsimile machines and personal digital assistants), either by wired connection using electrical conductors, or by wireless communication using infrared signals or LPRF signals. For many applications, LPRF signals are preferred, as they do not require line-of-sight between two devices being connected. LPRF communication devices have an LPRF module providing LPRF functionality. They transmit very weak radio signals compared to radio signals sent by cellular telephones such as GSM telephones. Thus, the LPRF communication devices are energy efficient and they have a short range and a high radio capacity.
Recently, LPRF systems have been proposed for providing communications between a plurality of transceivers through a short-range link having a coverage range of several meters. “Bluetooth” is one such LPRF systems. This system is designed to operate in an open (non-reserved) radio spectrum band around 2.4 gigahertz using Frequency Hopping Spread Spectrum (FHSS) system, with 79 channels each having a 1 MHz bandwidth. Bluetooth is targeted for communication devices which are located within an operable range of the LPRF system to communicate with each other.
In LPRF systems, particularly in Bluetooth systems, each of the two ends of an LPRF link (LPRF modules) assumes either one of two different states: master and slave. One end is always a master and the other end is a slave. These states are interchangeable so that the end that earlier was a slave can, on its request, become a master, whereas the earlier master becomes a slave.
In order to communicate over an LPRF link successfully, the two ends of the LPRF link, or two LPRF modules, must time their transmissions appropriately so that they each transmit only when the other one listens. This timing is controlled by the master, which synchronises the link. The link synchronisation refers to the fact that a frequency hopping scheme is defined by the address of the master and the timing by the master's clock. A data net of one master and one or more slaves is called a piconet. Data is exchanged in the piconet using Time Division Duplexing (TDD), in which there are predetermined master-to-slave and slave-to-master slots. Each of the slots has a duration of 625 μs. At simplest, single-slot communication mode is used, wherein each data packet takes 1 time slot. Additionally, two different multiple slot packet transmission modes are supported, where a packet occupies either 3 or 5 slots. With single-slot packets, the frequency is changed after each slot (after each packet) and in multiple slots packets after each multi-slot packet. In Bluetooth, two major types of data transmission links are used: Synchronous Connection-Oriented (SCO) links and Asynchronous Connection-Less (ACL) links. These have various sub-types, and in some (but not all) of them a receipt is provided after each burst, immediately following transmission of data for which the receipt is issued.
Bluetooth provides no centralised co-ordination of frequencies or timing of the transmissions between piconets. Thus, in the presence of multiple piconets, some collisions occur. These collisions are part of the normal operation of Bluetooth, but they are tolerable because there are up to 79 frequency channels and each of the piconets has its own frequency hopping scheme. The collisions are, therefore, rare enough. With ACL-links, retransmissions can also be used when collisions have happen, so that a collision does not cause severe problems.
Wireless relay networks also exist which, in effect, extend an operating range of a local RF system by using specific LPRF communication devices referred to as relay devices to interface with and provide communication between two or more user's communication devices. WO 98/17032 discloses a system in which many relay devices are wirelessly connected to each other to form an LPRF network. Each relay device has at least one, typically two LPRF modules in order to connect with at least one or two neighbouring relay devices, respectively.
In Bluetooth, each LPRF module (master) can serve up to seven active slaves. It is also possible for an LPRF communication device, particularly for a relay device, to have more than two LPRF modules. Thus, various types of links can be simultaneously in use by the same LPRF communication device. This increases the capacity of the LPRF system allowing a larger number of LPRF communication devices to simultaneously use the LPRF system. As mentioned in the foregoing, the link synchronisation is determined by the master. In other words, transmission time is decided for the slave modules by an LPRF communication module which is part of the relay device. As result, a number of LPRF modules of a single LPRF communication device may transmit and receive at different times. This interferes the receiving LPRF modules of said single LPRF communication device. The LPRF communication device can still operate, although with a reduced capacity, since most of transmissions interfering the receiving LPRF modules occur on frequency channels which differ from those listened by the receiving modules.
It is worth noting that in Bluetooth, unlike in GSM, for example, there are no different uplink and downlink frequency areas separated by a separation band. Instead, each of the frequency bands can be used in either direction. This enhances radio resource usage, but also makes it impossible to separate transmission branch of an LPRF module from a reception branch of the same LPRF module and explains why one LPRF module can either transmit or receive but not do the both on the same time. Additionally, different inter-modulation product signals may occur and interfere the receiver branch. A transmission of one LPRF module may thus block a reception by another LPRF module even though the frequencies of the transmission and reception would differ. The LPRF modules must be made so that they can endure rather strong connection to their receiving branches from neighbouring LPRF modules' transmitter branches.